In the polyurethane industry there is a need for delayed action catalysts, i.e. catalysts that will delay the onset (initiation time) of the isocyanate-polyol reaction (gelling reaction) or isocyanate-water reaction (blowing reaction) while not substantially affecting the time to the end of the reaction, or the final cure. Additionally, a need exists either to protect the amine catalyst from acidic additives such as halogenated flame retardants or, the converse, to provide efficient cure in the presence of such acidic additives.
In practice, the production of molded polyurethane foam articles requires the use of delayed initiation catalysts. They are necessary in order to fill the mold with the wet chemical foam composition and permit mold lid closing before the rising foam reaches the part line of the mold. Catalysts typically used for this purpose are acid blocked tertiary amines, that is to say tertiary amines 30-90 mole % neutralized with an acid.
The disadvantages of using such delayed initiation catalysts are (a) the formation of equilibrium salts with amine crosslinkers, which are used to provide stability and contribute load bearing in the foam product, resulting in unstable foam exemplified by very coarse cell structure and (b) the drift in reactivity with time that is shown by the underdevelopment of green strength at demold. Also, the use of acid-blocked amine catalysts necessary for processing efficiency greatly reduces the processing latitude available to the polyurethane producer.
As presently practiced, these delayed catalysts are tertiary amine compositions having some type of chemical adduct formation. Each type of adduct presents problems to the end user. For example, in acid or phenolic blocked amine catalysts, the substitution of stronger or weaker acids has been tried without success. The stronger acids extend the delay of initiation to an unacceptable time while the weaker acids do not provide sufficient delay. Reduction of tertiary amine catalyst levels is partially successful but at a cost of increased cure times or higher oven temperatures, both of which cause other operating problems.
Manufacturers of polyurethane foam typically use masterbatches, or premix compositions, consisting of one or more polyols plus water, amine catalysts and, optionally, cell stabilizers, organometallic salts, flowing agents, crosslinkers, dyes or pigments and flame retardants. These are known as the "B" side components. Depending on the size of the individual molding plant, the masterbatches may be consumed in as short as 2-4 hours or as long as 4-5 days. Some of the typically used polyol masterbatches containing an acid blocked amine catalyst package rapidly change in reactivity when used over several days. Typically after several hours storage, especially in water-containing masterbatches, this reactivity drift becomes apparent. Thus, low to moderate volume molders generally will consume their masterbatches over 2-4 days and experience the cure drift problem.
Also, within a few hours of using a masterbatch containing acid blocked amines the rising foam loses sufficient stability and begins to exhibit cell coarsening that leads to cell collapse, especially under the bleeder vents, producing very large cells as well as extending the cure time. This condition is known in the industry as "Monday morning foam."
A typical approach to solving this problem in the molding of polyurethane foam is to have a skeleton staff make the masterbatch on Sunday night before production starts or to make a short batch on Friday to prevent having any masterbatch stand over the weekend and losing reactivity.
U.S. Pat. No. 3,036,980 discloses curing organic resins and elastomers using a latent curing agent which includes a volatile curing compound that is released by heating or otherwise during a curing reaction. A zeolitic molecular sieve acts as a carrier for the curing compound.
U.S. Pat. No. 3,384,680 discloses that polyepoxides and polyurethanes may be cured by encapsulated curing agents. The curing agent is adsorbed on an adsorbent having internal pore structure and active pore sites. The absorbent is coated with a shielding agent adsorbed into the adsorbent. The shielding agent is intended to prevent the slow escape of the curing agent from the absorbent or its interaction with other components in the composition.
U.S. Pat. No. 3,417,046 discloses the use of a release agent to improve the release of active chemical curing agents previously absorbed in crystalline zeolitic molecular sieves. It is taught that water displaces the curing agent from the molecular sieve. Since water is so strongly absorbed by the molecular sieve, the curing agent is rapidly released leading to premature release effects (curing).
U.S. Pat. No. 3,499,864 discloses one-part polymer compositions comprising (a) a curable, liquid polythiol polymer, (b) a dormant amine activatable curing agent, (c) a desiccating, hydroscopic accelerating agent, and (d) an amine-loaded molecular sieve, which are characterized by excellent storage stability at temperate ambient temperatures (in the absence of moisture) and undergo relatively fast cures when exposed to moisture of a moisture-laden atmosphere, are improved and rendered storage stable at elevated ambient temperatures by containing in the compositions (e) a stabilizing amount of elemental sulfur. The desiccating agent is needed to prevent premature release of the amine from the molecular sieve by extraneous moisture.
U.S. Pat. No. 3,528,933 discloses a curable resin composition comprising a flowable, curable resin and a latent curing agent distributed throughout the resin within a solid, stable organic crystalline inclusion compound unreactive in the crystalline form with the resin. The inclusion compound is a crystalline host material and contains within the crystalline structure a guest material which will cure the resin when released from the crystalline structure by heat or other means.
U.S. Pat. No. 4,341,689 discloses a two component polyurethane coating system in which the pot life is extended (about six hours) and is cured by exposure to atmospheric moisture in about an hour or two. The cure is accelerated by the presence of an organotin accelerator. It is also taught that water rapidly replaces amine catalysts from molecular sieves. No water is contained within the formulations. A catalyst-molecular sieve intermediate is prepared by mixing diazo bicyclooctane (triethylenediamine) catalyst with a 13X zeolite (NaX zeolite).
U.S. Pat. No. 4,518,718 discloses a closed cell rigid polyurethane foam which contains up to about 60% by weight of molecular sieves capable of sorbing molecules with effective critical diameters of up to about 10 Angstroms. The molecular sieve component of the foam can be preloaded with catalysts or with reactive compounds that can be released upon activation of the foam to control and complete crosslinking after the foam is formed. There is no free water present in the foaming composition. In the presence of large quantities of molecular sieves or zeolites, as is the case in the '718 invention, all of the water is absorbed by these desiccants at the time foaming should occur. Consequently, satisfactory expansion of the reaction mixture is accomplished by incorporating into it alternate blowing agents such as the halocarbons.
U.S. Pat. No. 4,707,501 discloses a polyurethane foam catalyst composition that eliminates polyurea-containing polyol based masterbatch cure drift.
FR 2,587,710 discloses an anhydrous zeolite which does not absorb nitrogen, is used in a polyol composition as a moisture absorber and stabilizer which prevents excessive decreases in the pot life of mixtures of the polyol and a polyisocyanate during the preparation of solid polyurethanes and prevents water-promoted generation of carbon dioxide and the resulting formation of voids in the polyurethanes during polymerization.